When people say it flopped, they're talking relative to previous entries. I wouldn't personally put that label on, but i can understand why others would.
depends on your definition of the word "flop". Flop, at least to me, means a failure. Halo 5 didnt fail, considering it sold roughly 1 million in the USA week one. i would call it underperfoming, a more suiting word for it, considering the performance of past halo titles.